This invention relates to optical fiber filters and to the method of making such filters using tapered single-mode fibers. In particular, the invention provides a way of making wavelength filters with a sinusoidal response or modulated sine response having any desired filtering amplitude and period of oscillation.
Tapered optical fiber filters are well known in the art. They are made by tapering a single-mode optical fiber in such a way as to produce an interference between cladding modes, thereby creating a transmission which is wavelength dependent.
One such tapered fiber filter is described in Canadian Patent No. 1,284,282 issued May 21, 1991. It provides a passband filter comprising a plurality of successive biconical tapered portions on a single-mode fiber, such tapered portions having different profiles to produce the desired filtering characteristic.
Also, U.S. Pat. No. 4,946,250 of Aug. 7, 1990 by Gonthier et al., discloses a passband/stopband filter which is formed of two biconical tapers each having a given profile and being separated from each other by a small distance. This enables transmission of one signal of predetermined wavelength while stopping a second signal of a different wavelength.
The difficulty in such prior art filters is that the response usually involves more than two modes, thereby producing uncontrollable modulations in-the sine response. Moreover, in a standard single-mode matched cladding fiber operation, in the 1200 to 1700 nm range, the maximum amplitude of oscillation is less than 90%, resulting in filters restricted to small filtering amplitudes of 1 to 3 dB.
It is an object of the present invention to produce optical fiber filters having a sinusoidal response with essentially any desired period and any amplitude, from 1% to 99.9%.
Another object of the invention is to produce such filters with a single tapered section on the single-mode fiber.
Other objects and advantages of the invention will be apparent from the following description thereof.
The response of the filter of the present invention can be defined by the following relation:
T=1xe2x88x92xcex1sin2[(xcexxe2x88x92xcexo)Π/xcex9]
where:
T is the optical transmission of the filter;
xcex1 is the amplitude of the filter;
xcex is the wavelength of the light passing through the filter;
xcexo is a reference wavelength or center wavelength of the filter; and
xcex9 is the wavelength period of the filter.
In essence, according to the present invention, an optical fiber filter is provided which comprises an essentially adiabatic taper in a single-mode fiber, having an elongated central zone with a sloped portion at each end thereof , and on the slope at each end of the central zone there is provided a non-adiabatic taper, thereby forming two coupling regions at the extremities of the central zone, such as to produce a predetermined sinusoidal response in amplitude and wavelength period of the filter. This provides conditions at the extremities of the long adiabatic taper which excite LP01 and LP02 modes in a controlled fashion. When a large amplitude is desired, both modes need to be excited equally, i.e. 50% of the power must be coupled in the LP02 mode, whereas at first all power resides in the fundamental mode.
The preferred method of making such novel optical fiber filter is described below.
A single mode fiber is connected between a light source setup and a detector setup. The light source setup provides the ability to switch between a laser source and a broadband source, both of them operating in the 1550 nm window. The detector setup allows the light to be switched between a photodetector and a spectrum analyzer, so that the response at a given wavelength of the filter can be determined with the laser and the photodetector, whereas the spectral response of the filter can be monitored using the broadband source and the spectrum analyzer. Prior to commencing the operation, the light sources and the detectors are normalized so that the filter function would be relative to the resulting measurements.
The fiber is then stripped of its protective jacket over a predetermined length, for example 20 mm, and placed on a suitable fabrication setup on which the fiber is clamped at each end of the stripped section and which includes two motorized stages that can systematically pull on the fiber at each clamped end. Also the setup comprises a punctual heat source, such as a torch, which is mounted on a motorized three-axis holder allowing the flame to approach the fiber and to longitudinally brush it to simulate a wider flame.
The first step of the filter fabrication process is to produce a long essentially adiabatic taper on the fiber""s stripped section. For example, if the fiber is stripped of its protective coating over 20 mm, the adiabatic taper may be produced by heating the stripped section with the heat source, e.g. a torch with flame, and pulling it another 20 mm, thereby creating a reduction in diameter of about 50%. The flame of the torch is made to brush the fiber over a certain length, e.g. 6 mm or more, to produce the adiabatic condition of the taper, i.e. a taper that does not cause higher order cladding modes to be excited. Since no extra modes are excited at this stage, all power stays in the fundamental mode and the transmission power remains constant as a function of elongation produced by the pulling action. Once the adiabatic taper is finished, the torch is removed and the trace on the spectrum analyzer will show that there is no coupling, the transmission being at 0 dB.
The next fabrication steps create conditions at the extremities of the adiabatic taper produced in the first step, such as to excite the LP01 and LP02 modes in a controlled fashion. When a large amplitude is desired, both modes must be excited equally, i.e. 50% of the power must be coupled in the LP02 mode. Such coupling is realized by making a non-adiabatic taper, which is a short mixing taper, on the slopes at each end of the central zone of the adiabatic taper. For this purpose, a small flame is used with no brushing and the fiber is pulled until the power has decreased to the appropriate value, e.g. 50%. The power goes through cycles and it may be necessary to go through several cycles, e.g. 2 or 3, before reaching the 50% value. In order to produce a 50% coupling, and excite as little as possible the third mode LP03, the position of the non-adiabatic taper on each slope is critical. Such position may be determined by trial and error for various types of fibers, but when using a standard matched cladding fiber, such as SMF-28, made by Corning, the torch must be approached at a point on the slope where the diameter of the adiabatic taper is 68% of the fiber diameter. The flame size is then chosen to produce a short non-adiabatic taper with maximum amplitude coupling of around 50%.
After making the first non-adiabatic taper on one slope of the adiabatic taper, a second short non-adiabatic taper is made in like manner on the other slope to match the coupling of the first. This creates a filter structure which has two coupling regions at its extremities and a central beating region. In such structure the ratio between LP01 and LP02 can be readily controlled. As with a two-arm interferometer, if the power splitting is 50% at both ends of the device, the contrast will be maximum. This will be shown by the spectrum analyzer at successive elongation points during the formation of the second non-adiabatic taper. Once the contrast is maximum, the pulling process that forms the second non-adiabatic taper is stopped. A good indication that the two non-adiabatic tapers are matched, is the decrease of the excess loss at the transmission peaks of the filter as the elongation progresses, which loss becomes very low at the end. Thus, to achieve a desired total amplitude of the filter, one must produce non-adiabatic tapers on the slope of the adiabatic taper that are matched and have a splitting ratio of-half the total amplitude desired.
With this structure it is also possible to produce a filter with any desired period under 100 nm. Elongating the central zone of the filter, between the two non-adiabatic tapers, will increase the phase shift between the two modes and reduce accordingly the period of the filter. The transmission peak loss does not change with the period which means that changing the period is independent of the non-adiabatic tapers.
Finally, if a modulated sinusoidal response is desired, the size of the non-adiabatic or mixing tapers may be modified to make them more wavelength dependent, thus producing a modulation in the sine period of the filter.